This invention relates generally to color imaging and the use of plural exposure and development steps for such purposes and more particularly to the optimum use of black development steps.
One method of printing in different colors is to uniformly charge a charge retentive surface and then expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop (REaD) process may be repeated to subsequently develop images of different colors in superimposed registration on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). Each different image may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
In the creation of a "REaD" image on image (IOI) process color image the placement of the black toner development step in the process sequence creates some difficulty. Once black toner has been deposited, no further colors can be placed at that point in the image because the black toner absorbs subsequent exposure illumination. This can be an advantage when trying to mask registration errors in which case black first is desired, but it also limits the use of black for undercolor removal and for extending the gamut of dark yellow, magenta and red hues, which are advantages of developing black last. Black first, depositing black toner first in the imaging process, and black last, depositing black toner last in the imaging process, both have advantages in different portions of the color image. A solution is to employ black development twice during the creation of a color image; black first to provide masking in some portions of the image, then the primary colors, then black last in those portions of the image where black-on-color gives a color gamut superior to that of black next-to color.
An extension of having black developed twice in the image creation process is to have two black developing steps that take place at two different developing stations; one black developing station having a relatively high gloss toner and the other black developing station having a relatively low gloss toner. This would enable the desirable combination of low gloss text and high gloss pictorials on the same page which is a very desirable result.
Various types of printing machines have hereinbefore been used as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention.